Regulation of gene expression is linked to the organization of nuclear structure. The functional complexity of nuclear organization is reflected by the multiplicity of specialized nuclear substructures or regions that contribute to DNA replication and/or gene expression. Specialized regions found in most cell types include the nucleolus, RNA polymerase 11 transcription and processing domains, coiled bodies, PML domains, and Barr bodies (41-49).
A principal component of nuclear architecture is the filamentous ribonucleoprotein network known as the nuclear matrix, a developmentally modulated structure as reflected by its protein composition that is specific to tissue type and differentiation stage (30,31). The nuclear matrix is involved in gene localization, and also in the concentration and subnuclear localization of a broad spectrum of transcription regulatory factors required for cell type and tissue-restricted gene expression (30-35). For example, several DNA binding proteins have been identified which localize to and interact with the nuclear matrix, including SATB-1 (50), ARBP (51,52), Lamin B (53,54), NMP-1 (55) and NMP-2 (21,35).
One class of transcription regulatory factors which are key transactivators of tissue-specific genes of the hematopoietic and bone lineages are the AML (acute myeloid leukemia) family of proteins. Members of the human AML, class of proteins include AML-1 (Meyers et al. (1993) Mol. Cell. Biol. 13:6336-6345), AML-2 (Levanon et al. (1994) Genomics 23:425-432) and AML-3 (Levanon et al. (1994), supra.). These proteins are closely related to the murine polyomavirus enhancer core-binding proteins PEBP2.alpha.A and PEBP2.alpha.B, formerly identified as the murine leukemia enhancer CBF (core-binding factor) proteins. In fact, AML-3 is identical to its murine homolog PEBP2.alpha.A.
AML proteins and their closely related family members share a homologous DNA binding domain located in the C-terminus, referred to as the rhd (runt homology domain) based on its previous documentation in the Drosophila runt gene (12-18). Structurally, the AML proteins are all highly homologous, with greater than 90% homology within the rhd domain and greater than 60% identity overall (Meyers et al. (1996) Oncogene 13:303-312).
Subsequent to the cloning of AML-1, a larger alternatively spliced form of AML-1, AML-1B, was identified (Meyers et al. (1995) Mol Cell. Biol. 15(4):1974-1982). AML-1B contains additional sequences at the N- and C-termini, including a potent transactivation domain within the C-terminus. AML-1 and AML-3 (PEBP2.alpha.A) both bind the consensus enhancer core motif 5'-TGYGGT (Y=C or T) and heterodimerize with the non-DNA binding partner CBF-.beta. (1, 11-14).
One trait of particular interest among certain AML-1 proteins is their susceptibility to multiple chromosomal translocations in lymphoid and myeloid leukemias. The t(8; 21) translocation, which is present in 12-5% of acute myeloid leukemia cases (1-4), maintains the DNA binding domain of AML-1, but replaces the C-terminal domain with sequences from the MTG8 gene on chromosome 8. The rare t(3:2) translocation also removes sequences C-terminal to the AML-1 DNA binding domain (5-7). The t(12;21. translocation fuses the transcription factor TEL to the N-terminus of AML-1B (8-10). Thus, at least a subset of human leukemias is directly associated with, specific reorganization of the AML-1 coding) region.
An understanding of the normal molecular mechanisms that mediate the biological activities of AML-1 transcription factors, such as transcriptional activity and association with sub-nuclear compartments involved in transcription, would thus clearly benefit the art.